The present invention is directed to a process for preparing copolymers. More specifically, the present invention is directed to a process for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds. One embodiment of the present invention is directed to a process for preparing copolymers which comprises, in the order stated: (1) adding monomers containing unsaturated carbon-to-carbon bonds, a first polymerization initiator, a second polymerization initiator, and a solvent to a reaction vessel; (2) purging the resulting solution with an inert gas; (3) sealing the reaction vessel and pressurizing it by addition of an inert gas to a pressure of from about 20 to about 600 kilopascals above ambient atmospheric pressure; (4) maintaining the temperature within the pressurized reaction vessel at a temperature of from about 50 to about 100.degree. C. for a period of from about 60 to about 300 minutes; (5) thereafter maintaining the temperature within the pressurized reaction vessel at a temperature of from about 80 to about 115.degree. C. for a period of from about 30 to about 300 minutes, wherein the temperature in step 5 is higher than the temperature in step 4; and (6) subsequently maintaining the temperature within the pressurized reaction vessel at a temperature of from about 115 to about 160.degree. C. for a period of from about 30 to about 180 minutes, wherein the temperature in step 6 is higher than the temperature in step 5. Polymers prepared according to the process of the present invention can be particularly useful as softenable materials in migration imaging members.
Processes for preparing copolymers are known. For example, U.S. Pat. No. 2,757,166 (Segro et al.) discloses a process for the bulk polymerization of acrylonitrile or copolymerization of acrylonitrile with at least one other compound containing a polymerizable CH.sub.2 =C&lt; grouping. The process enables the polymerization or copolymerization of acrylonitrile to substantial completion in the presence of a catalyst such as tertiary butyl permaleic acid or tertiary butyl perphthalic acid. Typical reaction conditions include temperatures of from 85 to 130.degree. C. and pressure of about 1 atmosphere.
In addition, U.S. Pat. No. 4,141,806 (Keggenhoff et al.) discloses a bulk photopolymerization process for esters of acrylic and methacrylic acids. The ethylenically unsaturated monomers are polymerized in bulk in the presence of from 0 to 10 percent by weight of a photoinitiator by irradiation of UV light in a first reaction stage at or below the boiling point of the reaction mixture, up to a conversion of 40 to 80 percent by weight, followed by a second reaction stage at a temperature which has been raised by from 20 to 120.degree. C. and is above the glass transition temperature of the resulting polymer, up to a conversion of above 90 percent by weight, the percentages in each case relating to the total amount of monomer.
Of further background interest with respect to polymerization processes are U.S. Pat. Nos. 2,666,046, 2,846,424, 3,222,429, and 3,498,938.
Migration imaging members are well known, and are described in detail in, for example, U.S. Pat. No. 3,975,195 (Goffe), U.S. Pat. No. 3,909,262 (Goffe et al.), U.S. Pat. No. 4,536,457 (Tam), U.S. Pat. No. 4,536,458 (Ng), U.S. Pat. No. 4,013,462 (Goffe et al.), and "Migration Imaging Mechanisms, Exploitation, and Future Prospects of Unique Photographic Technologies, XDM and AMEN", P. S. Vincett, G. J. Kovacs, M. C. Tam, A. L. Pundsack, and P. H. Soden, Journal of Imaging Science 30 (4) July/August, pp. 183-191 (1986), the disclosures of each of which are totally incorporated herein by reference. Migration imaging members containing charge transport materials in the softenable layer are also known, and are disclosed, for example, in U.S. Pat. No. 4,536,457 (Tam) and U.S. Pat. No. 4,536,458 (Ng), the disclosures of each of which are totally incorporated herein by reference. A typical migration imaging member comprises a substrate, a layer of softenable material, and photosensitive marking material in the form of a fracturable layer contiguous with the upper surface of the softenable layer. The member is imaged by first electrically charging the member and exposing the charged member to a pattern of activating electromagnetic radiation such as light to form a latent image on the member. Subsequently, the imaged member is developed by one of several methods, such as application of heat, solvent, solvent vapor, or the like, causing the marking material in the exposed areas of the member to migrate in depth through the softenable material toward the substrate.
The expression "softenable" as used herein is intended to mean any material which can be rendered more permeable, thereby enabling particles to migrate through its bulk. Conventionally, changing the permeability of such material or reducing its resistance to migration of migration marking material is accomplished by dissolving, swelling, melting, or softening, by techniques, for example, such as contacting with heat, vapors, partial solvents, solvent vapors, solvents, and combinations thereof, or by otherwise reducing the viscosity of the softenable material by any suitable means.
The expression "fracturable" layer or material as used herein means any layer or material which is capable of breaking up during development, thereby permitting portions of the layer to migrate toward the substrate or to be otherwise removed. The fracturable layer is preferably particulate in the various embodiments of the migration imaging members. Such fracturable layers of marking material are typically contiguous to the surface of the softenable layer spaced apart from the substrate, and such fracturable layers can be substantially or wholly embedded in the softenable layer in various embodiments of the imaging members.
The expression "contiguous" as used herein is intended to mean in actual contact, touching, also, near, though not in contact, and adjoining, and is intended to describe generically the relationship of the fracturable layer of marking material in the softenable layer with the surface of the softenable layer spaced apart from the substrate.
The expression "optically sign-retained" as used herein is intended to mean that the dark (higher optical density) and light (lower optical density) areas of the visible image formed on the migration imaging member correspond to the dark and light areas of the illuminating electromagnetic radiation pattern.
The expression "optically sign-reversed" as used herein is intended to mean that the dark areas of the image formed on the migration imaging member correspond to the light areas of the illuminating electromagnetic radiation pattern and the light areas of the image formed on the migration imaging member correspond to the dark areas of the illuminating electromagnetic radiation pattern.
The expression "optical contrast density" as used herein is intended to mean the difference between maximum optical density (D.sub.max) and minimum optical density (D.sub.min) of an image. Optical density is measured for the purpose of this invention by diffuse densitometers with a blue Wratten No. 94 filter. The expression "optical density" as used herein is intended to mean "transmission optical density" and is represented by the formula: EQU D=log.sub.10 [l.sub.o /l]
where l is the transmitted light intensity and l.sub.o is the incident light intensity. For the purpose of this invention, all values of transmission optical density given in this invention include the substrate density of about 0.2 which is the typical density of a metallized polyester substrate used in this invention.
Various means for developing the latent images can be used for migration imaging systems. These development methods include solvent wash away, solvent vapor softening, heat softening, and combinations of these methods, as well as any other method which changes the resistance of the softenable material to the migration of particulate marking material through the softenable layer to allow imagewise migration of the particles in depth toward the substrate. In the solvent wash away or meniscus development method, the migration marking material in the light struck region migrates toward the substrate through the softenable layer, which is softened and dissolved, and repacks into a more or less monolayer configuration. In migration imaging films supported by transparent substrates alone, this region exhibits a maximum optical density which can be as high as the initial optical density of the unprocessed film. On the other hand, the migration marking material in the unexposed region is substantially washed away and this region exhibits a minimum optical density which is essentially the optical density of the substrate alone. Therefore, the image sense of the developed image is optically sign reversed. Various methods and materials and combinations thereof have previously been used to fix such unfixed migration images. In the heat or vapor softening developing modes, the migration marking material in the light struck region disperses in the depth of the softenable layer after development and this region exhibits D.sub.min which is typically in the range of 0.6 to 0.7. This relatively high D.sub.min is a direct consequence of the depthwise dispersion of the otherwise unchanged migration marking material. On the other hand, the migration marking material in the unexposed region does not migrate and substantially remains in the original configuration, i.e. a monolayer. In migration imaging films supported by transparent substrates, this region exhibits a maximum optical density (D.sub.max) of about 1.8 to 1.9. Therefore, the image sense of the heat or vapor developed images is optically sign-retained.
The background portions of an imaged member can sometimes be transparentized by means of an agglomeration and coalescence effect. In this system, an imaging member comprising a softenable layer containing a fracturable layer of electrically photosensitive migration marking material is imaged in one process mode by electrostatically charging the member, exposing the member to an imagewise pattern of activating electromagnetic radiation, and softening the softenable layer by exposure for a few seconds to a solvent vapor thereby causing a selective migration in depth of the migration material in the softenable layer in the areas which were previously exposed to the activating radiation. The vapor developed image is then subjected to a heating step. Since the exposed particles gain a substantial net charge (typically 85 to 90 percent of the deposited surface charge) as a result of light exposure, they migrate substantially in depth in the softenable layer towards the substrate when exposed to a solvent vapor, thus causing a drastic reduction in optical density. The optical density in this region is typically in the region of 0.7 to 0.9 (including the substrate density of about 0.2) after vapor exposure, compared with an initial value of 1.8 to 1.9 (including the substrate density of about 0.2). In the unexposed region, the surface charge becomes discharged due to vapor exposure. The subsequent heating step causes the unmigrated, uncharged migration material in unexposed areas to agglomerate or flocculate, often accompanied by coalescence of the marking material particles, thereby resulting in a migration image of very low minimum optical density (in the unexposed areas) in the 0.25 to 0.35 range. Thus, the contrast density of the final image is typically in the range of 0.35 to 0.65. Alternatively, the migration image can be formed by heat followed by exposure to solvent vapors and a second heating step which also results in a migration image with very low minimum optical density. In this imaging system as well as in the previously described heat or vapor development techniques, the softenable layer remains substantially intact after development, with the image being self-fixed because the marking material particles are trapped within the softenable layer.
The word "agglomeration" as used herein is defined as the coming together and adhering of previously substantially separate particles, without the loss of identity of the particles.
The word "coalescence" as used herein is defined as the fusing together of such particles into larger units, usually accompanied by a change of shape of the coalesced particles towards a shape of lower energy, such as a sphere.
Xeroprinting processes employing migration imaging members are also known. For example, U.S. Pat. No. 4,970,130 (Tam et al.), the disclosure of which is totally incorporated herein by reference, discloses a xeroprinting process which comprises (1) providing a xeroprinting master comprising (a) a substrate; and (b) a softenable layer comprising a softenable material, a charge transport material capable of transporting charges of one polarity, and migration marking material situated contiguous to the surface of the softenable layer spaced from the substrate, wherein a portion of the migration marking material has migrated through the softenable layer toward the substrate in imagewise fashion; (2) uniformly charging the xeroprinting master to a polarity opposite to the polarity of the charges that the charge transport material in the softenable layer is capable of transporting; (3) uniformly exposing the charged master to activating radiation, thereby discharging those areas of the master wherein the migration marking material has migrated toward the substrate and forming an electrostatic latent image; (4) developing the electrostatic latent image; and (5) transferring the developed image to a receiver sheet.
In addition, U.S. Pat. No. 4,883,731 (Tam et al.), the disclosure of which is totally incorporated by reference, discloses a xeroprinting process wherein the xeroprinting master is a developed migration imaging member wherein a charge transport material is present in the softenable layer. According to the teachings of this patent, the xeroprinting process entails uniformly charging the master to a polarity the same as the polarity of charges which the charge transport material is capable of transporting, followed by flood exposure of the master to form a latent image, development of the latent image with a toner, and transfer of the developed image to a receiving member. The contrast voltage of the electrostatic latent image obtainable from this process generally initially increases with increasing flood exposure light intensity, typically reaches a maximum value of about 45 to 50 percent of the initially applied voltage and then decreases with further increase in flood exposure light intensity. The light intensity for the flood exposure step thus generally must be well controlled to maximize the contrast potential.
U.S. Pat. No. 4,880,715 (Tam et al.), the disclosure of which is totally incorporated by reference, discloses a xeroprinting process wherein the xeroprinting master is a developed migration imaging member wherein a charge transport material is present in the softenable layer and non-exposed marking material in the softenable layer is caused to agglomerate and coalesce. According to the teachings of this patent, the xeroprinting process entails uniformly charging the master to a polarity the same as the polarity of charges which the charge transport material is capable of transporting, followed by flood exposure of the master to form a latent image, development of the latent image with a toner, and transfer of the developed image to a receiving member. The contrast voltage of the electrostatic latent image obtainable from this process generally initially increases with increasing flood exposure light intensity, typically reaches a maximum value of about 60 percent of the initially applied voltage and then decreases with further increase in flood exposure light intensity. The light intensity for the flood exposure step thus generally must be well controlled to maximize the contrast potential.
U.S. Pat. No. 4,853,307 (Tam et al.), the disclosure of which is totally incorporated herein by reference, discloses a migration imaging member containing a copolymer of styrene and ethyl acrylate in at least one layer adjacent to the substrate. When developed, the imaging member can be used as a xeroprinting master. According to the teachings of this patent, the xeroprinting process entails uniformly charging the master to a polarity the same as the polarity of charges which the charge transport material is capable of transporting, followed by flood exposure of the master to form a latent image, development of the latent image with a toner, and transfer of the developed image to a receiving member.
Preferred materials for the softenable layer of migration imaging members include copolymers of vinyl monomers, such as styrene-acrylic copolymers, including styrene-hexylmethacrylate or styrene-ethyl acrylate-acrylic acid copolymers, polystyrenes, including polyalphamethyl styrene, alkyd substituted polystyrenes, styrene-olefin copolymers, styrene-vinyltoluene copolymers, vinyl toluene butadiene copolymers, styrene butadiene copolymers, vinyl toluene acrylate copolymers, vinyl toluene .alpha.-methyl styrene copolymers, vinyl acetate polymers, saturated polyesters, unsaturated polyesters, mixtures thereof, copolymers thereof, and the like. While many of these materials are commercially available, not all commercially available vinyl copolymers exhibit characteristics that are preferred or desirable in softenable materials intended for use in migration imaging members. For example, one commercially available copolymer of styrene, ethyl acrylate, and acrylic acid containing about 74 mole percent styrene, about 25 mole percent ethyl acrylate, and about 1 mole percent acrylic acid, with a M.sub.n of about 30,000, a M.sub.w of about 72,000, an acid number of about 8, a T.sub.g of about 65.degree. C., and a melt viscosity of about 4.times.10.sup.5 poise at 110.degree. C. may exhibit undesirable characteristics, such as high melt viscosity as a result of the high styrene content; during heat development of an image on a migration imaging member with a softenable layer of a material with a melt viscosity of this magnitude, the softenable layer may not allow sufficient migration of the photosensitive marking material to form an image of acceptable optical contrast density. Further, if the imaged member is then used as a xeroprinting master, the lack of acceptable contrast density can result in insufficient electrostatic contrast voltage for xeroprinting. An imaging member containing a softenable layer of a high melt viscosity material may also exhibit other undesirable characteristics, such as reduced photosensitivity at the temperatures required to develop the member. Other commercially available copolymers of styrene, ethyl acrylate, and acrylic acid, such as one containing about 48 mole percent styrene, about 50 mole percent ethyl acrylate, and about 2 mole percent acrylic acid, with a M.sub.n of about 21,000, a M.sub.w of about 54,000, an acid number of about 15, a T.sub.g of about 36.degree. C., and a melt viscosity of about 2.times.10.sup.4 poise at 110.degree. C. may exhibit undesirable characteristics, such as a low glass transition temperature (Tg) as a result of the low styrene content of the polymer; the low Tg can lead to a tendency of imaging members containing this material as a softenable layer to block under conditions of elevated temperatures and pressures. Blocking or sticking together results when the imaging member is stored in roll or sheet form under pressure and under relatively high storage conditions (such as about 35.degree. C.), and can cause damage such as delamination of the layers of the imaging member when separation of the blocked roll or sheets is attempted.
Copolymers of vinyl monomers with highly desirable characteristics for use as softenable materials in migration imaging members are known. For example, U.S. Pat. No. 4,853,307, the disclosure of which is totally incorporated herein by reference, describes in Examples XVIII, XIX, and XX a process for preparing a terpolymer of styrene, ethyl acrylate, and acrylic acid. The process generally entails adding the monomers of styrene, ethyl acrylate, and acrylic acid to a reactor vessel containing a solvent, such as toluene. The monomers are allowed to equilibrate to the reactor's temperature, typically from about 70 to about 100.degree. C., while the system is purged by bubbling nitrogen gas in the monomer solution. The monomer solution is stirred during purging and subsequent polymerization. The initiator is added to another portion of the solvent and is allowed to dissolve in or mix with the solvent before it is added to the reaction vessel. The polymerization is then allowed to proceed for 5 to 7 hours while the temperature in the reactor is controlled by cooling. The process described in U.S. Pat. No. 4,853,307, however, has some disadvantages in that at the end of the synthetic procedure, the residual levels of styrene and ethyl acrylate remaining in the terpolymer solution can be undesirably high (typically over 7 percent by weight styrene and over 7 percent by weight ethyl acrylate remain in the solution). The presence of these monomers in the solution at relatively high concentrations can have several drawbacks. For example, styrene and ethyl acrylate may pose potential health and safety hazards. In addition, ethyl acrylate has a very low odor threshold; the odor of ethyl acrylate vapor is readily detectable at 1 part per million airborne concentration, and the odor is relatively strong and moderately irritating at 4 parts per million airborne concentration. Thus, high concentrations of ethyl acrylate in terpolymer solutions of styrene, ethyl acrylate, and acrylic acid used to form softenable layers for migration imaging members may present potential safety problems and undesirable conditions in the preparation area. Further, the relatively high concentrations of residual monomers in the solution can increase the cost of producing the polymer, since the residual monomers result in "dead weight" in the polymer solution. Further, for polymers containing residual styrene monomers or residual ethyl acrylate monomers, it is extremely difficult to remove the residual monomers from a coated film of the polymer solution, since both monomers have high boiling points (for styrene, 145.degree. C. and for ethyl acrylate, 99.degree. C.). If the residual monomers are not removed from the film, their vapor emission inside a vacuum coating chamber while migration marking particles are being vacuum evaporated onto the softenable layer can adversely affect the imaging properties of the resulting imaging member; vapor release inside the vacuum chamber can cause the background pressure to rise to a level at which functional vacuum is lost. Residual monomers in the coated polymer film can also lower the glass transition temperature of the polymer and cause the softenable layer to block during the vacuum evaporation process to apply migration marking material to the softenable layer. When the conditions for preparing a softenable layer from a solution of a polymer containing high residual monomers are adjusted by increasing the temperature of the drying zone, however, the resulting softenable layer frequently exhibits an "orange peel" appearance on the surface, which can affect the resolution of the printed images because of the uneven surface.
Accordingly, while known materials and processes are suitable for their intended purposes, a need remains for improved processes for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds. In addition, a need remains for processes for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds with high yields. Further, there is a need for processes for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds wherein the residual concentration subsequent to completion of the process of each monomer originally present in the reaction mixture is less than about 0.5 percent by weight. There is also a need for processes for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds wherein the resulting copolymers enable preparation of high quality migration imaging members capable of generating high quality images. A need also exists for processes for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds wherein the resulting copolymers enable preparation of high quality xeromasters capable of generating high quality images. In addition, there is a need for processes for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds wherein the processes reduce the amounts of residual monomers present in the solution. Further, there is a need for processes for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds wherein the processes reduce the amounts in the solution of residual monomers which may present potential health or safety problems. Additionally, there is a need for processes for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds that are cost efficient. A need also remains for processes for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds with improved reaction times. There is also a need for processes for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds with yields of over 99 percent. In addition, there is a need for processes for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds wherein the resulting copolymers are substantially free of gel formation, thereby preventing defects in migration imaging members employing the copolymer as a softenable material. Further, there is a need for processes for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds wherein the resulting copolymers exhibit acceptable melt viscosity and improved mechanical strength. Additionally, there is a need for processes for preparing copolymers of monomers having unsaturated carbon-to-carbon bonds wherein the resulting copolymers exhibit a polydispersity (M.sub.w /M.sub.n) of about 2 or greater (preferably 3 or greater) and a molecular weight (M.sub.w) of about 20,000 or greater.